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Roses for every day of the year
Growing roses year-round is exacting work in cold, dark Finland. New research has brought the necessary information on production techniques to commercial greenhouses, thanks to which high-quality roses can be grown without large surface areas.

Arja-Leena Paavola

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Roses have for centuries held a special place amongst other flowers. Their beautiful mildly scented flowers and thorny stems have become symbols of both religion and romance. In Europe roses have been grown in monastery yards since the Middle Ages, as they have an important symbolic position in the Roman Catholic Church.

“Roses have been grown commercially in Finland since at least 1847, when the price list of Finska Trädgårdsodlings Sällskapet (Finnish Horticultural Society) mentions 42 different greenhouse rose cultivars. Nowadays, cut roses are Finland’s most important commercial greenhouse flower. The lack of light in winter at Finland’s latitude, however, sets its own demands on producing a good harvest,” says Liisa Särkkä, Dr. Sc. (Agr.& For).

During the last two decades, growing roses has developed considerably. Surface farming has given way to limited growing media, leading to the elimination of soil-borne plant diseases and pests. Irrigation and fertilisation have also been made more effective. At the beginning of the 1990s, the use of artificial lighting began to spread, leading to year-round production.

To remain economical, production demands detailed planning. In her dissertation on horticulture, defended in February, Särkkä studied the best ways to improve the yield, quality and vase life of roses in year-round greenhouse production.

“Development of the greenhouse climate control has made possible the control, follow-up and regulation of the greenhouse conditions. In the mid-1990s, a new production technique, in which new outgrowing shoots from a cutting are bent at least 90 degrees at the base of the shoot, was introduced. The plant is left with plenty of leaf mass to ensure its good growth. The method has not completely replaced the traditional production method in Finland, but is nowadays common.”

Carbon dioxide for roses

To make year-round production economically profitable, good cultivars are also needed that can be profitably grown with limited growing media. Efficient lighting in the greenhouse, carbon dioxide enrichment and good production techniques are key factors, to say nothing of the grower’s professional skill.

Carbon dioxide is one of the most important growth factors in raising roses, as it is the basic element of assimilation. According to Särkkä, carbon dioxide enrichment is especially important in winter even though it raises costs.

“Greenhouses are not ventilated in winter and the carbon dioxide concentration easily drops below that of normal air. Then the plant growth slows down without supplementary enrichment. In summer the vents are open and the outside carbon dioxide flows into the greenhouse, but even then growth is more efficient if additional carbon dioxide is given. This point has also been proved in research into global climate change. The carbon dioxide concentration inside the plant stand is easily less than that of the air even in summer if the air stream is unable to circulate the air properly inside the vegetation,” Särkkä points out.

The best way to give plants carbon dioxide is to install the narrow distribution pipes close to the growing media, so the carbon dioxide is immediately available to the plants. The grower is also able, with the help of the correct air humidity and air temperature, to prevent harmful insects and plant diseases from spreading.

Särkkä did her research in the greenhouses of Agrifood Research Finland in Piikkiö. The initiative came from greenhouse growers, who actively followed progress of the work and its results throughout the research. The information was really needed by the growers.

“They also immediately put to use the accumulated knowledge in the research. For example, the information about the optimal amount of lighting and lighting times was adopted straight away. The production method presented in my research does not need large greenhouse areas per produced square metre crop. I was even surprised myself with the size and quality of the harvest, because we were aiming at short-stemmed flowers, which are more in demand in Finland than long-stemmed ones,” says Särkkä.

Roses need their beauty sleep

The research showed that certain rose cultivars need as much as six hours of darkness to last longer in the vase. The quality of the light is also of great significance. A rose simply cannot tolerate the light of the high-pressure sodium lamps, especially around the clock, without the leaf stomata functioning being disturbed. This way the rose may evaporate more than it can absorb from the vase and that is why the flowering shoot wilts.

A rose should last at least a week in a vase.

There are also differences between the cultivars in their suitability for year-round production. The red cultivar Mercedes, which was earlier much cultivated in Finland, does not tolerate winter growing as well as, for example, the yellow cultivar Frisco, which, on the other hand, lasted well in a vase.

But is it then sensible to grow roses year-round in a country that is so dark in winter?

“Year-round growing also guarantees year-round jobs. I think that is important. Actually, the roses are better quality in winter than in summer, because of the high summer temperatures, which cannot be controlled for the time being and so lower the quality. Certainly, lamps consume energy but fortunately they can be used in Finland to heat the greenhouses. For example, in the Netherlands, where it is much warmer, they must get rid of some of this lamp energy through ventilation.”

Before becoming a rose doctor, Särkkä studied biology at the University of Umeå in Sweden. On returning to Finland, she completed her degree in horticultural science at the University of Helsinki. Särkkä works as senior scientist at Agrifood Research Finland, Horticulture in Piikkiö. After roses, it will be the turn of other greenhouse plants.

“I have liked flowers all my life. I like nature in general and I think there is something indescribably magnificent in the beauty of flowers. They reflect the complexity of nature in the most beautiful way. Flowers always bring a great feeling of joy and I grow them myself. I have a garden where I have planted all kinds of perennials, as well as flowering bushes. I also have many houseplants, maybe a few too many.”

Särkkä cannot name her favourite rose off-hand. “There are so many rose cultivars. I like many of them for their colour and shape, and different-sized roses. Perhaps the most striking of all is the Kardinal with its large dark red flowers.”

Liisa Särkkä: Yield, quality and vase life of cut roses in year-round greenhouse production. Yliopistopaino 2004. 116 p., pdf 64 p. ISBN 952-10-2254-x, 952-10-2255-8 (pdf). http://ethesis.helsinki.fi/

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